The alkalinity value won't directly help you decipher the calcium and magnesium makeup of the water and the hardness value can't either. Hardness is primarily computed from the calcium and magnesium concentrations, but their relative amounts are not determinable from the raw hardness value. There are separate calcium and magnesium test kits available. But considering that the cost of water testing is low, it doesn't make a lot of sense to have your own test kits unless your water source quality varies. You only need a lab test suite that provides: calcium, magnesium, sodium, bicarbonate, sulfate, and chloride. Those are the primary constituents in most potable waters and these are the prime interest to brewing. I've seen lab testing services for under $20 per sample.
Originally Posted by Sean W.
If your water source is known to vary, then you should have a calcium test kit and an alkalinity test kit on hand since those are the primary contributors to mash pH performance. Reputable firms such as Hach or Lamotte are preferred testing kit providers. If your cities water supply comes from a consistent groundwater source or a large lake, there is probably little variability in the water quality. If a river is the source, then there may be more variability. If the city has several sources, then variability is likely and on-site testing is mandatory.